System and Methods for Heart Rate and Electrocardiogram Extraction from a Spinal Cord Stimulation System

ABSTRACT

A system and method for extracting a cardiac signal from a spinal signal include measuring a spinal signal at one or more electrodes that are connected to a neurostimulator and implanted within a patient&#39;s spinal canal and processing the spinal signal to extract the cardiac signal, which includes features that are representative of the patient&#39;s cardiac activity. Processing the spinal signal to extract the cardiac signal can include filtering the spinal signal, or use of model reduction schemes such as independent component analysis. The extracted cardiac signal can include a number of features that correspond to an electrocardiogram and can be used to determine the patient&#39;s heart rate and/or to detect a cardiac anomaly. Cardiac features that are determined from the cardiac signal can additionally be used to adjust parameters of the stimulation that is provided by the neurostimulator.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation application of U.S. patent application Ser. No.17/199,186, filed Mar. 11, 2021, which is a continuation application ofU.S. patent application Ser. No. 16/282,130, filed Feb. 21, 2019 (nowU.S. Pat. No. 10,974,042), which is a non-provisional application ofU.S. Provisional Patent Application Ser. No. 62/648,231, filed Mar. 26,2018. Priority is claimed to these applications, and they areincorporated herein by reference in their entireties.

FIELD OF THE TECHNOLOGY

Implantable stimulation devices deliver electrical stimuli to nerves andtissues for the therapy of various biological disorders. The presentapplication is related to a technique to extract data relating to apatient's cardiac activity from a Spinal Cord Stimulation (SCS) system.

INTRODUCTION

An SCS system typically includes an implantable medical device (IMD),or, more specifically, an implantable pulse generator (IPG) 10 shown inplan and cross-sectional views in Figures lA and 1B. The IPG 10 includesa biocompatible device case 30 that is configured for implantation in apatient's tissue that holds the circuitry and battery 36 (FIG. 1B)necessary for the IPG to function. The IPG 10 is coupled to electrodes16 via one or more electrode leads 14 that form an electrode array 12.The electrodes 16 are configured to contact a patient's tissue and arecarried on a flexible body 18, which also houses the individual leadwires 20 coupled to each electrode 16. The lead wires 20 are alsocoupled to proximal contacts 22, which are insertable into leadconnectors 24 fixed in a header 28 on the IPG 10, which header cancomprise an epoxy for example. Once inserted, the proximal contacts 22connect to header contacts 26 in the lead connectors 24, which are inturn coupled by electrode feedthrough pins 34 through an electrodefeedthrough 32 to circuitry within the case 30 (connection not shown).

In the illustrated IPG 10, there are thirty-two lead electrodes (E1-E32)split between four leads 14, with the header 28 containing a 2×2 arrayof lead connectors 24 to receive the leads' proximal ends. However, thenumber of leads and electrodes in an IPG is application specific andtherefore can vary. In a SCS application, the electrode leads 14 aretypically implanted proximate to the dura in a patient's spinal cord,and when a four-lead IPG 10 is used, these leads can be split with twoon each of the right and left sides. Two 16-eletcrode leads could alsobe used with each having a splitter allowing the leads to be connectedto two lead connectors 24. Each of the IPG's lead connectors 24 couldalso support different numbers of electrodes, such as 12 or 16electrodes. The proximal contacts 22 are tunneled through the patient'stissue to a distant location such as the buttocks where the IPG case 30is implanted, at which point they are coupled to the lead connectors 24.As also shown in FIG. 1A, one or more flat paddle leads 15 can also beused with IPG 10, and in the example shown thirty two electrodes 16 arepositioned on one of the generally flat surfaces of the head 17 of thepaddle lead, which surface would face the dura when implanted. In otherIPG examples designed for implantation directly at a site requiringstimulation, the IPG can be lead-less, having electrodes 16 insteadcarried by the case of the IPG for contacting the patient's tissue.

As shown in the cross section of FIG. 1B, the IPG 10 includes a printedcircuit board (PCB) 40. Electrically coupled to the PCB 40 are thebattery 36, which in this example is rechargeable; other circuitry 46coupled to top and/or bottom surfaces of the PCB 40, including amicrocontroller or other control circuitry necessary for IPG operation;and a charging coil 44 for wireles sly receiving a magnetic chargingfield from an external charger (not shown) for recharging the battery36. If battery 36 is permanent and not rechargeable, charging coil 44would be unnecessary.

The IPG 10 also includes one or more antennas 42 a and 42 b fortranscutaneously communicating with external programming devices, suchas a patient external controller 50 (FIG. 2 ) or a clinician programmer90 (FIG. 3 ), which is provided via the execution of software on aclinician programmer computer 96. While illustrated as a dedicateddevice, the patient external controller 50 can take the form of anapplication that resides on a smart device, such as smart phone or otherportable device. Antennas 42 a and 42 b are different in shape and inthe electromagnetic fields they employ. Telemetry antenna 42 a comprisesa coil, which can bi-directionally communicate with an external devicevia a magnetic induction communication link. Telemetry antenna 42 bcomprises a short-range Radio-Frequency (RF) antenna that operates inaccordance with a short-range RF communication standard, such asBluetooth, BLE, NFC, Zigbee, WiFi (802.11x), and the Medical ImplantCommunication Service (MICS) or the Medical Device RadiocommunicationsService (MDRS). As will be understood, external devices such as externalcontroller 50 and clinician programmer 90 include similar communicationcircuitry (e.g., antenna(s), modulation and demodulation circuitry,etc.) such that a communication link can be established between theexternal device and the IPG 10.

Implantation of IPG 10 in a patient is normally a multi-step process, asexplained with reference to FIG. 3 . A first step involves implantationof the distal ends of the lead(s) 14 or 15 with the electrodes 16 intothe epidural space of the spinal canal 60 of the patient through atemporary incision 62 in the patient's tissue 5. (Only two leads 14 withsixteen total electrodes 16 are shown in FIG. 3 for simplicity). Theproximal ends of the leads 14 or 15 including the proximal contacts 22extend externally from the incision 62 (i.e., outside the patient), andare ultimately connected to an External Trial Stimulator (ETS) 70. TheETS 70 is used during a trial stimulation phase to provide stimulationto the patient, which may last for two or so weeks for example. Tofacilitate the connection between the leads 14 or 15 and the ETS 70, ETSextender cables 80 may be used that include receptacles 82 (similar tothe lead connectors 24 in the IPG 10) for receiving the proximalcontacts 22 of leads 14 or 15, and connectors 84 for meeting with ports72 on the ETS 70, thus allowing the ETS 70 to communicate with eachelectrode 16 individually. Once connected to the leads 14 or 15, the ETS70 can then be affixed to the patient in a convenient fashion for theduration of the trial stimulation phase, such as by placing the ETS 70into a belt worn by the patient (not shown). ETS 70 includes a housing73 for its control circuitry, antenna, etc., which housing 73 is notconfigured for implantation in a patient's tissue.

The ETS 70 essentially mimics operation of the IPG 10 to providestimulation to the implanted electrodes 16, and thus includes contains abattery within its housing along with stimulation and communicationcircuitry similar to that provided in the IPG 10. Thus, the ETS 70allows the effectiveness of stimulation therapy to be verified for thepatient, such as whether therapy has alleviated the patient's symptoms(e.g., pain). Trial stimulation using the ETS 70 further allows for thedetermination of particular stimulation programs that seem promising forthe patient to use once the IPG 10 is later implanted into the patient.A stimulation program may include stimulation parameters that specifyfor example: which of the electrodes 16 are to be active and used toissue stimulation pulses; the polarity of those active electrodes(whether they are to act as anodes or cathodes); the current or voltageamplitude (A) of the stimulation pulses; the pulse width (PW) of thestimulation pulses; the frequency (f) of the stimulation pulses; theduty cycle (DC) of the stimulation pulses (i.e., the percentage of timethat the pulses are asserted relative to the period of the pulses) theshape of the stimulation waveform (e.g., one or more square pulses, oneor more ramped pulses, one or more sinusoidal pulses, or evennon-pulse-based waveforms, etc.); and other parameters related toissuing a burst of pulses, such as the number of pulses; etc.

At the end of the trial stimulation phase, a decision is made whether toabandon stimulation therapy, or whether to provide the patient with apermanent IPG 10 such as that shown in FIGS. 1A and 1B. Should it bedetermined that stimulation therapy is not working for the patient, theleads 14 or 15 can be explanted from the patient's spinal column 60 andincision 62 closed in a further surgical procedure. By contrast, ifstimulation therapy is effective, IPG 10 can be permanently implanted inthe patient as discussed above. (“Permanent” in this context generallyrefers to the useful life of the IPG 10, which may be from a few yearsto a few decades, at which time the IPG 10 would need to be explantedand a new IPG 10 implanted). Thus, the IPG 10 would be implanted in thecorrect location (e.g., the buttocks) and connected to the leads 14 or15, and then temporary incision 62 can be closed and the ETS 70dispensed with. The result is a fully-implanted stimulation therapysolution. If a particular stimulation program(s) had been determined aseffective during the trial stimulation phase, it/they can then beprogrammed into the IPG 10, and thereafter modified wirelessly, usingeither the external controller 50 or the clinician programmer 90.

An example of stimulation pulses as prescribed by a particularstimulation program and as executable by the IPG or ETS 70 isillustrated in FIG. 4 . In the example shown, each stimulation pulse isbiphasic, meaning it comprises a first pulse phase followed essentiallyimmediately thereafter by an opposite polarity pulse phase. The pulsewidth (PW) could comprise the duration of either of the pulse phasesindividually as shown, or could comprise the entire duration of thebiphasic pulse including both pulse phases. The frequency (f) andamplitude (A) of the pulses is also shown. Although not shown,monophasic pulses—having only a first pulse phase but not followed by anactive-charge recovery second pulse phase—can also be used.

Biphasic pulses are useful because the second pulse phase can activelyrecover any charge build up after the first pulse phase residing oncapacitances (such as the DC-blocking capacitors 107 discussed later) inthe current paths between the active electrodes. In the examplestimulation program shown in FIG. 4 , electrode E4 is selected as theanode electrode while electrode E5 is selected as the cathode electrode(during the first pulse phase), which because two electrodes 16 areimplicated, comprises what is known as bipolar stimulation. The pulsesas shown comprise pulses of constant current, and notice that theamplitude of the current at any point in time is equal but opposite suchthat current injected into the patient's tissue by one electrode (e.g.,E4) is removed from the tissue by the other electrode (E5). Notice alsothat the area of the first and second pulse phases are equal, ensuringactive charge recovery of the same amount of charge during each pulsephase. Although not shown, more than two electrodes can be active at anygiven time. For example, electrode E4 could comprise an anode providinga +10 mA current pulse amplitude, while electrodes E3 and E5 could bothcomprise cathodes with −7 mA and −3 mA current pulse amplitudesrespectively. Biphasic pulses are particularly beneficial when pulsesare issued at higher frequencies, although they may be used at lowerfrequencies as well.

The stimulation program executed by the IPG 10 and ETS 70 can be set oradjusted via a communication link from the external controller 50 (FIG.2 ) or clinician programmer 90 (FIG. 3 ). While the external controller50's antenna is usually within its housing, the clinician programmer 90may include a communication head or wand 94 containing an antenna andwired to computer 92. Further details concerning the clinicianprogrammer 90 may be as described in U.S. Patent Application Publication2015/0360038, and further details concerning an external controller canbe found in U.S. Patent Application Publication 2015/0080982. As isknown, both of the external communication devices have graphical userinterfaces that can be used by the clinician or patient to set andadjust the stimulation program that the IPG 10 or ETS 70 will run.

SUMMARY

A system is disclosed comprising measurement circuitry configured tomeasure a spinal signal at one or more electrodes that are connectableto a neurostimulator and implantable within a patient's spinal canal;and processing circuitry configured to process the spinal signal toextract a cardiac signal that comprises one or more features that arerepresentative of the patient's cardiac activity. At least one of themeasurement circuitry or the processing circuitry may be within theneurostimulator. The system may further include control circuitryconfigured to control stimulation circuitry to provide electricalstimulation to neural tissue, and the control circuitry may be furtherconfigured to adjust parameters of the electrical stimulation based onone or more properties of the cardiac signal.

The processing circuitry may be further configured to process the spinalsignal by performing a first filtering operation using a low-passfilter; and performing a second filtering operation using a movingaverage filter. The processing circuitry may be further configured toprocess the spinal signal by extracting a subcomponent of the spinalsignal using a model reduction scheme. The model reduction scheme mayinclude independent component analysis.

The spinal signal may include a monopolar spinal signal that is measuredas a differential voltage between one of the electrodes and a referencevoltage. The spinal signal may include a bipolar spinal signal that ismeasured as a differential voltage between two of the electrodes. Thespinal signal may include an arithmetic combination of two or morebipolar spinal signals that are each measured as a differential voltagebetween two of the electrodes, and the two or more bipolar spinalsignals may be selected to obtain a desired directionality of the spinalsignal.

The processing circuitry may be configured to determine the patient'sheart rate based on the cardiac signal. The processing circuitry may befurther configured to detect a cardiac anomaly based on the cardiacsignal, and the system may further include control circuitry configuredto communicate an indication of the cardiac anomaly to the patient.

A system is disclosed comprising a non-transitory computer-readablemedium comprising instructions to cause first control circuitry in acomputing device to present a graphical user interface that isconfigured to receive one or more first user settings associated withextracting, from a spinal signal, a cardiac signal that comprises one ormore features that are representative of a patient's cardiac activity;second control circuitry in an implantable medical device, the secondcontrol circuitry configured to measure a spinal signal at one or moreelectrodes connectable to the implantable medical device and implantablewithin a patient's spinal canal; and third control circuitry configuredto process the spinal signal to extract the cardiac signal.

The second control circuitry in the implantable medical device may befurther configured to control stimulation circuitry to provideelectrical stimulation to neural tissue. The graphical user interfacemay be further configured to receive one or more second user settingsassociated with adjusting parameters of the electrical stimulation basedon the extracted cardiac signal. Processing the spinal signal to extractthe cardiac signal may be based on the one or more first user settings,wherein the one or more first user settings may include a selection ofone of a plurality of cardiac signal extraction techniques. The one ormore of the plurality of cardiac signal extraction techniques may causethe second control circuitry in the implantable medical device toprocess the spinal signal by performing a first filtering operationusing a low-pass filter; and performing a second filtering operationusing a moving average filter. The one or more of the plurality ofcardiac signal extraction techniques may cause the second controlcircuitry in the implantable medical device to process the spinal signalby extracting a subcomponent of the spinal signal using a modelreduction scheme. The model reduction scheme may include independentcomponent analysis.

Measuring the spinal signal at one or more electrodes may be based onthe one or more first user settings, wherein the one or more first usersettings comprise one or more electrode settings that specify the one ormore electrodes that are used to measure the spinal signal. The one ormore electrode settings may specify a monopolar spinal signal that ismeasured as a differential voltage between one of the electrodes and areference voltage. The one or more electrode settings may specify abipolar spinal signal that is measured as a differential voltage betweentwo of the electrodes. The one or more electrode settings may specify anarithmetic combination spinal signal that is composed of two or morebipolar spinal signals that are each measured as a differential voltagebetween two of the electrodes, and the two or more bipolar spinalsignals may be selected to obtain a desired directionality of the spinalsignal.

The third control circuitry may be further configured to determine thepatient's heart rate based on the cardiac signal. The third controlcircuitry may be further configured to detect a cardiac anomaly based onthe cardiac signal. The third control circuitry may be furtherconfigured to communicate an indication of the cardiac anomaly to thepatient. The third control circuitry may be in the implantable medicaldevice.

A method is disclosed comprising measuring a spinal signal at one ormore electrodes that are connectable to a neurostimulator andimplantable within a patient's spinal canal; and processing the spinalsignal to extract a cardiac signal that comprises one or more featuresthat are representative of the patient's cardiac activity.

A system is disclosed comprising measurement circuitry configured tomeasure a spinal signal at one or more electrodes that are connectableto a neurostimulator and implantable within a patient's spinal canal;and processing circuitry configured to process the spinal signal usingone or more of a low-pass filter, a moving average filter, or a modelreduction scheme to extract a cardiac signal that comprises one or morefeatures that are representative of the patient's cardiac activity. Atleast one of the measurement circuitry or the processing circuitry maybe within the neurostimulator. The system may further include controlcircuitry configured to control stimulation circuitry to provideelectrical stimulation to neural tissue, and the control circuitry maybe further configured to adjust parameters of the electrical stimulationbased on one or more properties of the cardiac signal.

The processing circuitry may be further configured to process the spinalsignal by performing a first filtering operation using the low-passfilter; and performing a second filtering operation using the movingaverage filter. The processing circuitry may be further configured toprocess the spinal signal by extracting a subcomponent of the spinalsignal using the model reduction scheme. The model reduction scheme mayinclude independent component analysis.

The spinal signal may include a monopolar spinal signal that is measuredas a differential voltage between one of the electrodes and a referencevoltage. The spinal signal may include a bipolar spinal signal that ismeasured as a differential voltage between two of the electrodes. Thespinal signal may include an arithmetic combination of two or morebipolar spinal signals that are each measured as a differential voltagebetween two of the electrodes, and the two or more bipolar spinalsignals may be selected to obtain a desired directionality of the spinalsignal.

The processing circuitry may be configured to determine the patient'sheart rate based on the cardiac signal. The processing circuitry may befurther configured to detect a cardiac anomaly based on the cardiacsignal, and the system may further include control circuitry configuredto communicate an indication of the cardiac anomaly to the patient.

A method is disclosed comprising measuring a spinal signal at one ormore electrodes that are connectable to a neurostimulator andimplantable within a patient's spinal canal; and processing the spinalsignal using one or more of a low-pass filter, a moving average filter,or a model reduction scheme to extract a cardiac signal that comprisesone or more features that are representative of the patient's cardiacactivity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B respectively show an Implantable Pulse Generator (IPG)in plan and cross sectional views.

FIG. 2 shows a hand-held external controller for communicating with anIPG.

FIG. 3 shows a clinician programming system for communicating with anIPG or an External Trial Stimulator (ETS).

FIG. 4 shows example pulses in a stimulation program.

FIG. 5 shows an improved IPG (or ETS) that includes control circuitryprogrammed with a cardiac activity extraction algorithm, and furtherincluding sensing circuitry for sensing electrical signals at theimplanted electrodes in accordance with an example of the disclosure.

FIG. 6 shows an example spinal signal that might be measured at one ormore electrodes that are implanted within a patient's spinal column inaccordance with an example of the disclosure. FIG. 6 additionally showsvarious constituents of the spinal signal.

FIG. 7 shows the frequency distribution of an example spinal signal inaccordance with an example of the disclosure.

FIG. 8 shows a process for extracting a cardiac signal from a spinalsignal that is measured at one or more electrodes that are implantedwithin a patient's spinal column in accordance with an example of thedisclosure.

FIG. 9 shows a process for extracting a cardiac signal from a spinalsignal that is measured at one or more electrodes that are implantedwithin a patient's spinal column in accordance with an example of thedisclosure.

FIG. 10 shows an example paddle lead and the directions of standardelectrocardiogram (ECG) limb leads and augmented limb leads inaccordance with an example of the disclosure.

FIG. 11 shows example bipolar spinal vectors, the combination of thebipolar spinal vectors, and the extracted cardiac signals associatedwith the bipolar spinal vectors and the combination spinal vector inaccordance with an example of the disclosure.

FIGS. 12 through 14 show example bipolar spinal vectors and thecombination of the bipolar spinal vectors in accordance with an exampleof the disclosure.

FIG. 15 shows example cardiac signals that were extracted from measuredspinal signals and a simultaneously-measured ECG signal in accordancewith an example of the disclosure.

FIG. 16 shows an example graphical user interface that can be used toconfigure the manner in which an IPG measures spinal signals, extractscardiac signals from the measured spinal signals, and evaluates theextracted cardiac signals to provide information, alerts, or stimulationadjustments in accordance with an example of the disclosure.

FIG. 17 illustrates a representative computing environment on whichsoftware that provides an interface for configuring the manner in whichcardiac signals are extracted from measured spinal signals may beexecuted in accordance with an aspect of the disclosure.

DETAILED DESCRIPTION

While the primary function of the SCS electrodes 16 is to deliverelectrical stimulation, the electrodes 16 can also be used to senseelectrical activity in the area around the spinal cord. The inventorshave determined that this electrical activity includes information aboutthe patient's cardiac activity, which can be extracted from the sensedelectrical activity as described below.

FIG. 5 is a block diagram that illustrates an example of the componentsin an improved IPG 100 that includes sensing capability. In manyaspects, the IPG 100 mirrors the IPG 10 (e.g., includes communicationcircuitry to communicate with external devices such as externalcontroller 50 and/or clinician programmer 90, connects to electrodeleads in the same manner, etc.), but unlike IPG 10, IPG 100 includesadditional circuitry to enable sensing at its connected electrodes 16.Although described in the context of an IPG 100, it should be realizedthat the disclosed technique could also be operable in anyneurostimulator, such as an External Trial Stimulator 170 that generallymimics the operation of IPG 100.

The IPG 100 (or ETS 170) includes control circuitry 102 into which acardiac activity extraction algorithm can be programmed. Controlcircuitry 102 may comprise a microcontroller such as Part Number MSP430,manufactured by Texas Instruments. Other types of control circuitry maybe used in lieu of a microcontroller as well, such as microprocessors,FPGAs, DSPs, or combinations of these, etc. Control circuitry 102 mayalso be formed in whole or in part in one or more Application SpecificIntegrated Circuits (ASICs), as described in U.S. Patent ApplicationPublication 2012/0095529 and U.S. Pat. Nos. 9,061,140 and 8,768,453.

In the IPG 100 (or ETS 170) a bus 118 provides digital control signalsto one or more Digital-to-Analog converters (DACs) 104, which are usedto produce currents or voltages of prescribed amplitudes (A) for thestimulation pulses, and with the correct timing (PW, f). As shown, theDACs include both PDACs, which source current to one or more selectedanode electrodes, and NDACs, which sink current from one or moreselected cathode electrodes. In this example, a switch matrix 106 undercontrol of bus 116 is used to route the output of one or more PDACs andone or more NDACs to any of the electrodes, which effectively selectsthe anode and cathode electrodes. The control circuitry 102 thuscontrols stimulation circuitry (e.g., DACs 104 and switch matrix 106)via control signals provided over the buses 118 and 116 to providestimulation to a patient's neural tissue. The illustrated circuitry forproducing stimulation pulses and delivering them to the electrodes ismerely one example. Other approaches may be found for example in U.S.Pat. Nos. 8,606,362 and 8,620,436. Note that a switch matrix 106 isn'trequired, and instead a PDAC and NDAC can be dedicated to (e.g., wiredto) each electrode.

Notice that the current paths to the electrodes 16 include theDC-blocking capacitors 107 alluded to earlier, which provide additionalsafety by preventing the inadvertent supply of DC current to anelectrode and to a patient's tissue. As discussed earlier, capacitancessuch as these can become charged as stimulation currents are provided,providing an impetus for the use of biphasic pulses.

As described above, in addition to providing stimulation, the electrodes16 can be used to sense the electrical activity in the area of thespinal cord, and thus each electrode is further coupleable to at leastone sense amp 110. In the example shown, there are four sense amps 110each corresponding to a particular timing channel in which stimulationcan be issued. Under control by bus 114, a multiplexer 108 can coupleany of the electrodes to any of the sense amps 110 at a given time. Thisis however not strictly necessary, and instead each electrode can becoupleable to its own dedicated sense amp 110, or all electrodes can beselected for sensing at different times and presented by MUX 108 to asingle sense amp 110. The measured analog waveform, which may representa differential voltage between two electrodes 16 (a bipolar spinalsignal) or between a single electrode 16 and a reference voltage such asthe IPG 100's case (a monopolar spinal signal), is preferably convertedto a digital spinal signal 130 by one or more Analog-to-Digitalconverters (ADC(s)) 112, which may sample the waveform at 50 kHz forexample. The ADC(s) may also reside within the control circuitry 102,particularly if the control circuitry 102 has A/D inputs.

Notice that connection of the electrodes 16 to the sense amp(s) 110preferably occurs through the DC-blocking capacitors 107, such thatcapacitors are between the electrodes 16 and the sense amp(s) 110. Thisis preferred so as to not undermine the safety provided by theDC-blocking capacitors 107. The digitized spinal signal 130 is providedto the control circuitry 102, which processes the spinal signal 130 inaccordance with a cardiac activity extraction algorithm 132. The controlcircuitry 102 is thus configured to measure a spinal signal at one ormore electrodes that are connected to the IPG 100. As used herein, aspinal signal refers to a signal that is measured via one or moreelectrodes that are implanted within the epidural space of a patient'sspinal column. While cardiac activity extraction is described andillustrated as being performed in the digital domain, processing of thespinal signals 130 could also be performed via analog devices directlyon the measured analog signals (i.e., before the signals are processedby the ADCs 112). The circuitry illustrated in FIG. 5 includesmeasurement circuitry for measuring a spinal signal 130, processingcircuitry for extracting a cardiac signal from the spinal signal 130,stimulation circuitry for delivering electrical stimulation to thepatient's neural tissue via the electrodes 16, and control circuitry forcontrolling these different functional circuits. The circuitry forperforming these various functions may be described based on thecircuitry's specific functionality (e.g., measurement circuitry,processing circuitry, etc.) or more generally as control circuitry.

FIG. 6 shows an example of a spinal signal 130 over a 75 ms duration.The components that make up the spinal signal 130 include stimulationartifacts, evoked compound action potentials (ECAPs), spontaneous spinalcord fluctuations, cardiac signals, and possibly other types of signals.Stimulation artifacts are observed as a result of the electric fieldcreated in the tissue when stimulation is provided via electrodes 16. AnECAP is the cumulative response of neural fibers that are recruited as aresult of the electric field generated through stimulation. Spontaneousspinal cord fluctuations are not fully understood, but they are believedto be the result of numerous different nervous systems communications.The inventors have determined that the cardiac signals containinformation regarding the patient's cardiac activity and can beextracted from the spinal signal 130.

FIG. 7 shows the frequency distribution of an example spinal signal 130.The spinal signal 130 is primarily comprised of higher frequencycomponents than the frequency components of the cardiac signal, whichare primarily between 0 and 50 Hz, and in the case of high frequency ECGpotentials may be up to 500 Hz. Thus, an initial step in a process 180(FIG. 8 ) to extract the cardiac signal from the spinal signal 130,which process 180 may form part of the cardiac activity extractionalgorithm 132, is to process the spinal signal 130 using a low passfilter 150. In one embodiment, the low pass filter has a pass bandcutoff of 80 Hz and a stop band cutoff of 100 Hz, but it will beunderstood that these filter settings can be adjusted to obtaindifferent results. The resulting filtered spinal signal 132 includespeaks that correspond to the patient's heart rate. While the peaks inthe filtered signal 132 provide information regarding the patient'sheart rate, the portions of the signal between the peaks are relativelynoisy as a result of low frequency components in the signal 130 that maybe related to cardiac activity, wander baseline movement, or power linenoise. To reduce random components typical from movement wander noise,the filtered signal 132 is processed using a moving average filter 152.In one embodiment, the moving average filter 152 averages 30 samplepoints from the signal 132 to derive each point in the cardiac signal134 for a 50 kHz sampling rate of the original spinal signal 130, butother numbers of sample points could also be used. As shown in FIG. 8 ,the random noise between the peaks in the filtered signal 132 is absentin the cardiac signal 134, which exposes additional informationregarding the patient's cardiac activity as described below.

FIG. 9 shows an alternative process 182, which may form part of thecardiac activity extraction algorithm 132, for extracting a cardiacsignal from a spinal signal 130. The process 182 differs from theprocess 180 in FIG. 8 in that independent component analysis (ICA) isemployed on various signals in the process. As is known, independentcomponent analysis is a statistical technique used to extractsubcomponents from a multivariate signal. The cardiac signal is asubcomponent of the spinal signal 130 and can be extracted throughindependent component analysis. In the example shown, independentcomponent analysis is employed on the spinal signal 130 to extract thecardiac subcomponent and a moving average filter is applied (154) to theextracted subcomponent to obtain cardiac signal 134′″. Similarly,independent component analysis is employed on the filtered signal 132 toextract the cardiac subcomponent and a moving average filter is applied(156) to the extracted subcomponent to obtain cardiac signal 134″.Likewise, independent component analysis is employed (158) on thecardiac signal 134 itself to further isolate the cardiac subcomponent toobtain cardiac signal 134′. As will be understood, the process 182 neednot include every independent component analysis step. For example, theprocess 182 may include one of the paths through one of ICA & movingaverage filter block 154, ICA & moving average filter block 156, or ICAblock 158. As will be understood, the different paths produce differentresults and require different computational resources, so selecting adesired path involves a balance of these parameters. While independentcomponent analysis is illustrated, it will be understood that principalcomponent analysis (PCA), singular value decomposition (SVD), properorthogonal decomposition (POD), or other model reduction techniques mayalso be employed as they are all model reduction schemes that aim toreduce measured data into smaller sets of data key relevant components.In the remainder of this Specification, a cardiac signal or an extractedcardiac signal refers to a cardiac signal extracted from a spinal signalthrough any of the above-described processes or other signal extractionprocesses. As illustrated by the processes 180 and 182, the controlcircuitry 102 of the IPG 100 is configured to process a spinal signal130 to extract a cardiac signal 134 that comprises one or more featuresthat are representative of the patient's cardiac activity, whichfeatures are visible in the cardiac signal 134 without furtherprocessing as described below.

The inventors have observed that the cardiac signals 134 that areextracted from monopolar spinal signals 130 for certain lead-basedelectrodes 16 do not exhibit the same prominent cardiac features thatare observed in the cardiac signals 134 that are extracted frommonopolar spinal signals 130 for other lead-based electrodes 16. It isbelieved that these differences in the extracted cardiac signals 134that are associated with different electrodes 16 occur as a result ofthe physical positioning of the electrodes relative to the reference aswell as common mode noise. Thus, in some instances better results can beobtained through the extraction of cardiac signals 134 from bipolarspinal signals 130 (i.e., voltage differentials between two lead-basedelectrodes 16), which extraction can be accomplished in the same mannersas described above.

A bipolar spinal signal 130 is obtained in a similar manner as amonopolar spinal signal 130 with the exception that the inputs to asense amplifier 110 are selected (e.g., via the multiplexer 108) to bethe two desired lead-based electrodes 16 as opposed to a singlelead-based electrode 16 and a reference voltage. The bipolar spinalsignal can then be processed in the same manner as described above(e.g., via the processes 180 or 182) to extract a cardiac signal 134.Because spinal signals 130 and their extracted cardiac signal 134counterparts are vectors having a common direction, and because thelocation of electrodes 16 are known relative to each other, theelectrodes 16 that comprise a bipolar spinal signal 130 can be selectedto obtain a cardiac signal 134 having a desired directionality.

FIG. 10 illustrates a paddle lead 15 and the directionality of the limband augmented limb leads of a standard 12-lead ECG (note that leaddirection assumes that the paddle lead 15 is viewed from the posteriorto anterior direction). Assuming the paddle lead 15 is aligned with noangular deflection with respect to the patient's anatomical midline, acardiac signal 134 that is extracted from the bipolar spinal signal 130that is measured between electrodes E1 and E25 (E25-E1) would match thedirectionality of the Lead I limb lead of a standard ECG. Note thatsimilar results would be expected for bipolar spinal signals 130 forother pairs of laterally aligned electrodes (e.g., E2 and E26, E3 andE27, etc.). Similarly, a cardiac signal 134 that is extracted from abipolar spinal signal 130 comprised of vertically-aligned electrodes(e.g., E1 and E8, E9 and E16, etc.) would match the directionality ofthe augmented vector foot (aVF) limb lead of a standard ECG. Theelectrodes 16 that comprise bipolar spinal signals 130 can similarly beselected to approximate the directionality of the other illustratedstandard limb leads. While a paddle lead 15 is illustrated in FIG. 10 ,it will be understood that different electrodes can be selected acrossseparate leads (e.g., separate percutaneous leads 14) to obtain bipolarspinal signals 130 having the desired directionality. Moreover, wheremultiple leads are implanted in different anterior-posterior planes(e.g., as verified via a lateral fluoroscope), electrodes 16 can beselected to obtain bipolar spinal signals 130 (and associated extractedcardiac signals 134) that match the direction of one or more of thestandard ECG precordial leads.

Spinal signals 130 can also be arithmetically combined (i.e., added orsubtracted) to provide additional benefits in terms of the quality ofextracted cardiac signals 134 as well as directional selectivity. FIG.11 illustrates a cardiac signal 134A that is extracted from a bipolarspinal signal 130A that represents the voltage differential betweenelectrodes E18 and E1 and a cardiac signal 134B that is extracted from abipolar spinal signal 130B that represents the voltage differentialbetween electrodes E1 and E8. The cardiac features are much lessprominent in the cardiac signal 134A than in the cardiac signal 134B dueto the proximity of electrodes E1 and E18. When the signals 130A and130B are added, the resulting combined spinal signal 130C approximates(in magnitude and direction) the voltage differential between electrodesE18 and E8. The cardiac signal 134C that is extracted from the combinedspinal signal 130C is perhaps cleaner than either signal 134A or 134B.In fact, combined spinal signals 130 can often be used to extractsignals 134 and choose the ones with lesser noise. For example, thespinal signal 134C that is extracted from the combined spinal signal130C may be less noisy than the cardiac signal 134 that is extractedfrom the bipolar spinal signal 130 that represents the voltagedifferential between electrodes E18 and E8.

This technique is especially valuable when a cardiac signal having adesired directionality can only be obtained directly from a bipolarspinal signal 130 between electrodes that are closely positioned.Consider, for example, the extraction of a cardiac signal 134 having adirectionality that is aligned with the Lead I limb lead (FIG. 10 ). Asdescribed above, such a cardiac signal 134 can be extracted from abipolar spinal signal 130 comprised of electrodes that are laterallyaligned such as E1 and E25. However, due to the design of the lead 15,any pair of electrodes 16 that are laterally aligned are also positionedrelatively close to each other (e.g., E1 and E25, E2 and E26, etc.). Asillustrated by the signal 134A in FIG. 11 , the cardiac features in acardiac signal 134 that is extracted from a bipolar spinal signal 130comprised of electrodes that are in close proximity are less prominent.Thus, as shown in FIG. 12 , a more desirable cardiac signal 134 havingthe same directionality as Lead I might be extracted, for example, fromthe addition of a first bipolar spinal signal 130D representing thevoltage differential between electrodes El and E32 and a second bipolarspinal signal 130E representing the voltage differential betweenelectrodes E8 and E25. The resulting combined spinal signal 130F has thedesired directionality of Lead I, but it does not suffer from the closeproximity of electrodes that would be required to measure a similarspinal signal directly.

Combined spinal signals 130 can additionally represent a directionalitythat cannot be obtained through a direct bipolar spinal signal (e.g.,where no pair of electrodes is aligned with a desired directionality).As illustrated in FIG. 13 , for example, a cardiac signal 134 having adirectionality that is aligned with the augmented vector right (aVR)lead can be extracted from a combined spinal signal 130I that is createdfrom the addition of bipolar spinal signal 130G that represents thevoltage differential between electrodes E22 and E1 and bipolar spinalsignal 130H that represents the voltage differential between electrodesE30 and E8. While combinations of two bipolar spinal signals 130 havebeen described, it will be understood that combined spinal signals maybe created from greater numbers of constituent spinal signals 130. Thus,a cardiac signal 134 having a more precise directionality can beextracted from a combined spinal signal 130 than could be extracted froma bipolar spinal signal 130 comprised of any pair of electrodes. Cardiacsignals 134 can be extracted from combined spinal signals 130 in thesame manner as described above for monopolar and bipolar spinal signals130 (e.g., via processes 180 or 182). While spinal signals 130 may becombined prior to cardiac signal extraction, the constituent spinalsignals 130 (e.g., the constituent bipolar spinal signal pairs) can beprocessed to extract cardiac signals 134 and those cardiac signals 134can be combined to obtain a combined cardiac signal 134, which wouldhave the same directionality as a cardiac signal 134 that is extractedfrom the combined spinal signal 130 resulting from the combination ofthe constituent spinal signals 130. While FIGS. 11-13 illustrated anddescribed combined spinal signals in the context of a paddle lead 15, itwill be understood that combined spinal signals can be similarlyextracted from electrodes that are positioned on different types ofleads such as the separate percutaneous leads 14 that are illustrated inFIG. 14 .

Having described the manner in which cardiac signals 134 can beextracted from spinal signals 130 of different types, FIG. 15illustrates cardiac signals 134M and 134N, which were extracted from twodifferent monopolar spinal signals 130M and 130N, in comparison with asimultaneously-recorded ECG signal 136. As is clearly visible in FIG. 15, each of the signals 134 includes many of the same features as the ECGsignal 136. This shows that the extracted cardiac signals 134 carry muchof the same information as a traditional ECG. Although the features ofthe cardiac signals 134 differ slightly from the corresponding featuresof the ECG signal 136 (e.g., in amplitude, shape, and/or time), in theenlarged portion of FIG. 15 corresponding features between the ECGsignal 136 and the cardiac signals 134 are labeled using standard ECGlabels. As illustrated, each of the extracted cardiac signals 134includes portions that mirror the QRS complex in the ECG signal 136.Specifically, the extracted cardiac signals 134 exhibit a decrease,increase, and decrease in amplitude that corresponds to the similarpattern in the Q, R, and S waves in the ECG signal 136. The extractedcardiac signals 134 also exhibit increases in amplitude that correspondto the P and T waves in the ECG signal 136.

As is known, ECG signals such as 136 provide significant informationregarding a patient's cardiac activity, and thus the similar patterns inthe extracted cardiac signals 134 provide much of the same information.The most elementary information that can be derived from the extractedcardiac signal is the patient's heart rate. The heart rate can bedetermined based on the time between common features in an extractedcardiac signal 134, which common features represent correspondingcardiac activities for different heartbeats. For example, the heart ratecan be identified based on the time between consecutive ‘R wave’ peaksin a cardiac signal 134 or, stated differently, the number of such peaksin a given time period. Other cardiac parameters such as respirationrate and anomalies such as atrial fibrillation, arrhythmia, ventricularhypertrophy, myocardial infarction, myocardial ischemia, etc. can bedetected through more complex analyses of features in the extractedcardiac signal 134 such as QRS amplitude, QRS duration, and ST segmentelevation or depression.

These cardiac parameters can be calculated automatically using cardiacanalysis algorithms applied to one or more extracted cardiac signals134, which algorithms may form part of the cardiac activity extractionalgorithm 132. The automatically-determined cardiac parameters providemany useful benefits to the patient. A large number of patients that areimplanted with spinal cord stimulation devices are in an elevated agebracket in which cardiac monitoring is beneficial. Such patients willgreatly benefit from the ability to monitor for cardiac anomalies usingthe IPG 100 itself. In one embodiment, the IPG 100 may be configured tocommunicate an indication of a detected cardiac anomaly to an externaldevice such as external controller 50. If the external device isconnected to a wide-area network, the indication can be furthercommunicated by the device such as to the patient's physician.

Because the perception of pain affects a patient's cardiac activity, theautomatically-detected cardiac parameters such as heart rate andrespiration rate provide insight regarding the degree of pain that apatient is experiencing. In one embodiment, the IPG may adjuststimulation parameters based on changes in the detected cardiacparameters. For example, as heart rate and/or respiration rate increase,the IPG 100 may increase stimulation parameters according to apredefined relationship. In an alternate embodiment, the IPG 100 mayprompt a patient (e.g., via a communication to an external device suchas external controller 50) to adjust stimulation parameters when thedetected cardiac parameters increase or decrease by a predeterminedamount.

In one embodiment, the IPG 100 may include a motion detector such as anaccelerometer or a gyroscope such that the motion of the IPG 100 can beassociated with the detected cardiac parameters. Evaluation of themotion sensor enables the IPG 100 to determine whether the patient isengaging in physical activity and to correlate physical activity withthe detected cardiac parameters. Based on this established correlation,the IPG 100 can then determine when the cardiac parameters deviate froma normal range for a given level of physical activity such that thestimulation parameters can be automatically adjusted or the patient canbe prompted to adjust the stimulation parameters.

FIG. 16 illustrates a portion of an example graphical user interface 200that is provided on a clinician programmer 90 to establish cardiacactivity detection parameters. The graphical user interface 200 isrendered via the execution of computer program instructions by theclinician programmer computer 96, for example, and the interface 200 isconfigured to receive one or more user settings for extracting a cardiacsignal as described below. The illustrated portion of the GUI 200includes fluoroscopic image 202, which shows the one or more implantedleads relative to anatomical structures, such as vertebrae. Using theillustrated interface, a user can select a representation of theimplanted electrode lead from an electrode lead portion of the interface200 (not shown), which includes representations of various types of leadproducts such as a 1×8 percutaneous lead, a 1×16 percutaneous lead, anda 4×8 paddle lead. The user can then drag the lead representation 206for the one or more implanted leads onto the fluoroscopic image 202 andmanipulate its size and orientation until it aligns with the implantedelectrode lead in the image 202. Because the representations 206 areprogrammed with properties of the lead such as electrode size, shape,and spacing, the positioning of a lead representation 206 on thefluoroscopic image 202 relates the locations of the electrodes 16 to theimage 202. This enables the software operating on the clinicianprogrammer 90 to understand the location of the electrodes 16 withrespect to anatomical features such as the anatomical midline. This isparticularly useful for visualizing the anatomical location ofstimulation for a given set of stimulation parameters, but theelectrodes' anatomical locations can also be used in configuring cardiacactivity detection parameters.

The cardiac activity detection interface 204 includes multipleinterfaces for configuring cardiac activity detection parameters. Thecardiac electrode selector 210 enables the user to select either manualelectrode configuration or automatic electrode configuration to specifythe electrode settings that will be communicated to the IPG 10 tomeasure a spinal signal. When manual electrode configuration isselected, the user can access a manual electrode configuration interface212 that enables the user to manually select the electrodes 16 that areused to measure spinal signals 130 from which cardiac signals 134 areextracted. In the illustrated embodiment, the user can manuallyconfigure up to four different cardiac channels (i.e., up to fourdifferent electrode configurations to produce different cardiac signals134), but it will be understood that the graphical user interface 200and the IPG 100 may be configured to accommodate more or fewer cardiacchannels. For each cardiac channel, the user is provided with an optionto select a monopolar, bipolar, or combination electrode arrangement.Based on the selected type of electrode arrangement, the manualelectrode configuration interface 212 enables the user to select desiredelectrodes. For example, in the illustrated embodiment, the user hasselected a bipolar electrode arrangement for cardiac channel 1. Based onthis selection, the manual electrode configuration interface 212 enablesthe user to select the positive and negative electrodes that willcomprise the bipolar spinal signal 130 from which the cardiac signal 134for cardiac channel 1 will be extracted. The user may make theseselections by entering (e.g., typing an electrode identifier) thedesired electrodes in the appropriate fields in the interface 212 or byselecting the desired electrodes on the lead representation 206 thatoverlays the fluoroscopic image 202. In the illustrated embodiment, theselected electrodes are highlighted and the directionality of the spinalsignal 130 is illustrated on the lead representation 206. When theelectrode parameters to be used for cardiac activity detection have beencommunicated to the IPG 100 and the IPG 100 is implementing theparameters, the extracted cardiac signal 134 for the selected channel iscommunicated to the CP programmer and is displayed in the interface 212.This enables the user to evaluate the quality of the cardiac signal 134that is obtainable for the current selections in near real time. In theillustrated embodiment, a simultaneously-recorded ECG is alsoillustrated in the interface 212 to enable a comparison of the extractedsignal.

When automatic electrode configuration is selected, the user can accessan automatic electrode configuration interface 214 that is substantiallysimilar to the manual electrode configuration interface 212. Theautomatic electrode configuration interface 214, however, enables theuser to select from one of several predefined cardiac leads. In theillustrated embodiment, the interface 214 enables selection of one ofthe six standard limb and augmented limb leads for each of the fourcardiac channels, but it will be understood that other predefinedcardiac signal types could be made available for selection. When theuser selects an automatic configuration, the software executing on theclinician programmer (“CP”) 90 uses the known anatomical location of theelectrodes to determine the particular electrode arrangement (e.g.,monopolar, bipolar, or combination) that most closely matches thedirectionality of the selected configuration. For example, the CPsoftware accounts for any angular offset of the one or more implantedleads (based on the lead representations 206) to identify an electrodearrangement that is most closely aligned with the directionality of theselected configuration. When multiple different electrode arrangementshave directionalities that are the same or similar to the selectedconfiguration, the CP software determines the most appropriate electrodearrangement from this group. In one embodiment, the CP software may beconfigured to select the most appropriate electrode arrangementaccording to a set of rules that are based on assumptions andpreferences (e.g., preference given to electrodes that are spacedfurther apart, etc.). In another embodiment, the CP software may beconfigured to select the most appropriate electrode arrangement byrequesting extracted cardiac signals 134 for the different electrodearrangements from the IPG 100 and evaluating the received signals todetermine the most appropriate electrode arrangement (e.g., determiningwhich electrode arrangement results in the highest quality extractedcardiac signal 134). After the appropriate electrode arrangement isdetermined, the electrode parameters may be transmitted to the IPG 100and the extracted cardiac signal 134 that is associated with thedetermined parameters may be displayed in the interface 214 forcomparison with a simultaneously-recorded ECG in the same manner as inthe interface 212 (note that no extracted cardiac signal is shown in theinterface 214 in the illustrated embodiment because cardiac channel 1 isshown as being selected to manual configuration). While the illustratedinterface 204 indicates that either manual or automatic electrodeconfiguration can be selected, in one embodiment, the two differentelectrode configuration types can be mixed on a cardiac channel basis(e.g., cardiac channel 1 may include a manual configuration whilecardiac channel 2 includes an automatic configuration, etc.). Theconfigured cardiac channels can be used in combination to determine thecardiac parameters (e.g., heart rate, heart rate variability, ST segmentelevation, respiration rate, etc.) and to detect cardiac events.

The cardiac extraction interface 204 additionally includes a cardiaccollection frequency selector 216 and a cardiac storage trigger (eventdetection trigger, schedule time of the day, patient trigger, or other).The selector 216 enables the user to determine how frequently cardiacsignals 134 are extracted from measured spinal signals 130. For example,cardiac signals 134 may be extracted and evaluated continuously, for a15 second period every minute, for a 15 second period every 5 minutes,for a 15 second period every 15 minutes, etc. As will be understood,because cardiac signal extraction requires significant processing,increased frequency results in increased energy use and thus shorterbattery life in the IPG 100.

The extracted cardiac signals 134 and associated detected cardiac eventscan be stored in a memory within the IPG 100 for later retrieval by anexternal device such as external controller 50 or clinician programmer90. The cardiac storage frequency selector 218 enables the user todetermine the duration of cardiac signals that should be maintained inthe IPG 100's memory. For example, the user may select to store the mostrecent 10 minutes, the most recent 30 minutes, the most recent one hour,etc. of each of the extracted cardiac signals (i.e., the cardiac signalassociated with each cardiac channel). The cardiac signals may bedownsampled before being stored to save memory, and, in one embodiment,downsampling settings may also be customizable via settings in thecardiac activity detection interface 204. When the selected cardiacstorage limit is reached, older portions of recorded cardiac signals 134may be deleted from the IPG 100's memory as more recent portions arestored.

The cardiac event storage selector 220 enables the user to select thenumber of cardiac events that are stored in the IPG 100's memory (e.g.,the 20 most recent events, the 50 most recent events, the 100 mostrecent events, etc.). The data associated with cardiac events mayinclude a text identification (e.g., tachycardia, bradycardia, etc.) andthe date, time, and related value (e.g., 147 beats per minute) of thedetected event. In one embodiment, the cardiac event may also include ashort segment (e.g., 15 seconds) of the one or more cardiac signals 134that led to the detection regardless of whether the one or more signalsare additionally stored as part of the cardiac signal storage. Just aswith the cardiac signal storage, when the number of events in the IPG100's memory reaches the selected value, the data associated with olderevents may be deleted from the IPG 100's memory as more recent eventsare stored.

The cardiac event alerts selector 222 enables the user to define whichevents result in alerts being communicated to an external device such asexternal controller 50. When the user selects to define event alertsusing the selector 222, an event alert definition interface 224 isdisplayed. The event alert definition interface 224 enables the user toselect the types of events (e.g., tachycardia) for which an alert shouldbe communicated to an external device and, for the selected types ofevents, to select a parameter limit (e.g., >150 beats per minute) atwhich the event alert should be communicated. In one embodiment, thetypes of events and associated limits that are selected via the eventalert definition interface 224 define the events and associated limitsthat result in events being stored in the IPG 100's memory. In anotherembodiment, the events that are stored in memory are set by default andthe event alerts and associated limits are treated separately as definedin the event alert definition interface 224.

The cardiac stimulation adjustment selector 226 enables the user tospecify whether and how stimulation parameters are to be adjusted basedon extracted cardiac signals. When the user selects to enablestimulation adjustment using the selector 226, a stimulation adjustmentdefinition interface 228 is displayed. The stimulation adjustmentdefinition interface 228 enables the user to select whether stimulationadjustments should be made automatically or whether the user should beprompted to approve stimulation adjustments based on detected cardiacparameters. The interface 228 additionally enables the user to selectthe maximum amount by which stimulation parameters can be adjusted basedon detected cardiac parameters. For example, in the illustratedembodiment, the user has selected a maximum stimulation amplitudeadjustment of 50% of the baseline stimulation amplitude and a maximumstimulation frequency adjustment of 100%. The interface 228 additionallyenables the user to select the cardiac parameter range over which thestimulation adjustments are implemented. In the illustrated embodiment,the stimulation adjustments are to be initiated when the detected heartrate is 30% above the average heart rate and at the maximum values whenthe detected heart rate is 100% above the average hear rate where theaverage heart rate may be programmable or extracted over a predefined orprogrammable time window. Based on the examples in the illustratedembodiment, assuming a baseline stimulation amplitude of 4.0 mA and anaverage heart rate of 80 beats per minute, the stimulation amplitudewould increase according to a programmable function (e.g., linear,exponential, piecewise function or other) from 4.0 mA to 6.0 mA as thedetected heart rate increased from 104 to 160 beats per minute.Similarly, assuming a baseline stimulation frequency of 400 Hz, thestimulation frequency would increase according to a programmablefunction from 400 Hz to 800 Hz as the detected heart rate increased from104 to 160 beats per minute. As will be understood, the stimulationparameter adjustments could also be defined in different ways and therelationship between the detected cardiac parameter and the stimulationadjustment parameter could be an inverse relationship as opposed to thedirect relationship examples given. If the user selects the patientprompt selector in the interface 228, the patient would be prompted(e.g., via a communication to the external controller 50) to accept astimulation adjustment based on detected cardiac parameters. As notedabove, stimulation adjustment may also incorporate input from a motionsensor such as an accelerometer, and, in such an embodiment, settingsthat specify the manner in which the motion sensor is utilized instimulation adjustments based on detected cardiac parameters may bespecified within the interface 228.

The cardiac detection type selector 230 enables the user to specify themanner in which cardiac signals 134 are to be extracted from measuredspinal signals 130. For example, the selector 230 may list a number ofextraction techniques such as portions of the processes 180 and 182 andvarious settings of the different processing blocks in those processesto enable the user to select and customize an extraction technique.Using the cardiac detection type selector 230 in conjunction with theelectrode selector 210 and electrode configuration interfaces 212 and214 enables the user to determine the particular settings that producethe highest quality extracted cardiac signals 134, which settings willdiffer from patient to patient.

The cardiac detection energy use indicator 232 provides a representationof the relative energy use associated with the selected cardiacdetection settings. As will be understood, the various selected cardiacdetection settings (e.g., cardiac detection type, quantity andconfiguration of the cardiac channels, cardiac collection frequency,etc.) influence the processing requirements for carrying out the desiredcardiac detection, which, in turn, influences the amount of energy thatwill be utilized by the IPG 100 to implement the settings. The cardiacdetection energy use indicator 232 enables the user to balance thequality of the extracted cardiac signals 134 (e.g., as visualizedthrough the electrode configuration interfaces 212 or 214) with theamount of energy required to obtain the desired cardiac signals 134. Inthe illustrated embodiment, the cardiac detection energy use indicator232 is presented as a bar that represents the relative amount of energyuse for cardiac activity detection from a minimum value to a maximumvalue. It will be understood that energy use can be depicted indifferent ways as well. The CP software is configured to communicate thesettings that are configured via the cardiac activity detectioninterface 204 to the IPG 100 via a communication link between theclinician programmer 90 and the IPG 100. Upon receiving the usersettings, the IPG measures one or more spinal signals 130 and extractsone or more cardiac signals 134 based upon the received settings.

FIG. 17 illustrates the various components of an example CP computer 96that may be configured to execute CP software for providing, forexample, the graphical user interface 200. The CP computer 96 caninclude a processor 322, memory 324, storage 320, graphics hardware 326,communication interface 330, user interface adapter 332 and displayadapter 334—all of which may be coupled via system bus or backplane 336.Memory 324 may include one or more different types of media (typicallysolid-state) used by the processor 322 and graphics hardware 326. Forexample, memory 324 may include memory cache, read-only memory (ROM),and/or random access memory (RAM). Storage 320 may store media, computerprogram instructions or software (e.g., CP software), preferenceinformation, device profile information, and any other suitable data.Storage 320 may include one or more non-transitory computer-readablestorage mediums including, for example, magnetic disks (fixed, floppy,and removable) and tape, optical media such as CD-ROMs and digital videodisks (DVDs), and semiconductor memory devices such as ElectricallyProgrammable Read-Only Memory (EPROM), Electrically ErasableProgrammable Read-Only Memory (EEPROM), and USB or thumb drive. Memory324 and storage 320 may be used to tangibly retain computer programinstructions or code organized into one or more modules and written inany desired computer programming language. As will be understood, the CPsoftware (i.e., the CP software that is executable to present the GUI200 and to communicate the received user settings to the IPG 100) may bestored on a medium such as a CD or a USB drive, pre-loaded on acomputing device such as the CP computer 96, or made available fordownload from a program repository via a network connection.Communication interface 330 may be used to connect the CP computer 96 toa network. Communications directed to the CP computer 96 may be passedthrough a protective firewall 338. Such communications may beinterpreted via web interface 340 or voice communications interface 342.Illustrative networks include, but are not limited to: a local networksuch as a USB network; a business' local area network; or a wide areanetwork such as the Internet. User interface adapter 332 may be used toconnect a keyboard 344, microphone 346, pointer device 348, speaker 350and other user interface devices such as a touch-pad and/or a touchscreen (not shown). Display adapter 334 may be used to connect display304 and printer 352. Processor 322 may include any programmable controldevice. Processor 322 may also be implemented as a custom designedcircuit that may be embodied in hardware devices such as applicationspecific integrated circuits (ASICs) and field programmable gate arrays(FPGAs). The CP computer 96 may have resident thereon any desiredoperating system.

While the GUI 200 has been described in terms of its presentation on aclinician programmer 90, it will be understood that a similar interfacethat enables similar parameter selections may be provided via executionof software on a different type of external device, which device cantake the form of a dedicated device (e.g., external controller 50) or anapplication residing on a smart phone or other personal device that mayinclude various ones of the components described with respect to FIG. 17. Likewise, although spinal signal measurement and processing to extractcardiac signals has been described as being performed on theneurostimulator (e.g., the IPG 100 or ETS 170), it may alternatively beexecuted on an external device such as the CP computer 96 (e.g., as partof computer program instructions that are executed by the CP computer),external controller 50, or another personal device that includes anapplication for performing the described processes. In such anembodiment, the external device may include measurement circuitry tomeasure spinal signals (e.g., instruct the IPG 100 or ETS 170 to measurespinal signals and communicate the digitized spinal signals 130 to theexternal device) and processing circuitry to process the spinal signals130 that are received from the neurostimulator to extract cardiacsignals 134. Such measurement and processing circuitry may also bereferred to as control circuitry.

While various specific embodiments and applications have been describedfor purposes of illustration, numerous modifications and variationscould be made by those skilled in the art without departing from thescope of the invention set forth in the claims.

What is claimed is:
 1. A method usable with a medical device comprisingelectrodes implantable within a patient's spinal canal, the methodcomprising: providing stimulation to the patient's spinal cord via afirst one or more of the electrodes; measuring a spinal signal at asecond one or more of the electrodes, wherein the spinal signalcomprises a neural response to the electrical stimulation; determining acardiac signal indicative of the patient's cardiac activity; andadjusting the stimulation at the medical device based on the cardiacsignal.
 2. The method of claim 1, further comprising processinginformation indicative of the spinal signal to determine the cardiacsignal.
 3. The method of claim 2, wherein the information is processedusing one or more of a low-pass filter, a moving average filter, or amodel reduction scheme.
 4. The method of claim 2, wherein the spinalsignal further comprises a stimulation artifact resulting from anelectric field created in response to the stimulation.
 5. The method ofclaim 2, wherein the spinal signal is measured at sense amplifiercircuitry in the medical device.
 6. The method of claim 5, wherein thespinal signal is measured using two of the second one or moreelectrodes.
 7. The method of claim 2, wherein processing the informationto extract the cardiac signal occurs at control circuitry in the medicaldevice.
 8. The method of claim 1, further comprising determining a heartrate of the patient based on the cardiac signal.
 9. The method of claim1, wherein determining the cardiac signal occurs at control circuitry inan external device in communication with the medical device.
 10. Themethod of claim 9, further comprising displaying the cardiac signal atthe external device, and/or displaying one or more features of thecardiac signal at the external device.
 11. A method usable with amedical device comprising electrodes implantable within a patient'sspinal canal, the method comprising: providing stimulation to thepatient's spinal cord at a first one or more of the electrodes;receiving information indicative of a spinal signal measured at a secondone or more of the electrodes, wherein the spinal signal comprises aneural response to the stimulation; and processing the information toextract a cardiac signal indicative of the patient's cardiac activity.12. The method of claim 11, wherein the information is processed usingone or more of a low-pass filter, a moving average filter, or a modelreduction scheme.
 13. The method of claim 11, wherein the spinal signalfurther comprises a stimulation artifact resulting from an electricfield created in response to the stimulation.
 14. The method of claim11, wherein the spinal signal is measured at sense amplifier circuitryin the medical device.
 15. The method of claim 14, wherein the spinalsignal is measured using two of the second one or more electrodes. 16.The method of claim 14, wherein processing the information to extractthe cardiac signal occurs at control circuitry in the medical device.17. The method of claim 11, further comprising adjusting the stimulationat the medical device based on the cardiac signal.
 18. The method ofclaim 11, further comprising determining a heart rate of the patientbased on the cardiac signal.
 19. The method of claim 11, whereinprocessing the information to extract the cardiac signal occurs atcontrol circuitry in an external device in communication with themedical device.
 20. The method of claim 19, further comprisingdisplaying the cardiac signal at the external device, and/or displayingone or more features of the cardiac signal at the external device.